Passive ranging to a target reflecting solar radiation

ABSTRACT

A passive ranging system measured spectra of solar radiation off an adjacent spot and a distal target. Reflected solar radiation and differential attenuation are used to estimate target range. A comparison of absorption spectra from the solar illuminated distal target compared to the adjacent location is performed. Since the sun&#39;s position is always known, the increased absorption due to a distant target is derived from the differential between the adjacent spot and the distal target.

RELATED APPLICATION(S)

This application is a Continuation-In-Part (CIP) of U.S. patentapplication Ser. No. 08/949,503 entitled “Passive Ranging to Source ofknown Spectral Emission”, filed Oct. 14, 1997 now U.S. Pat. No.6,222,018 which is a CIP of Ser. No. 08,506,847 filed Jul. 25, 1995 nowU.S. Pat. No. 5,677,761, the entire teachings of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

This invention relates to a measurement of the range of a source ofelectromagnetic radiation and, more particularly, to the use of passiveranging by examination of relative attenuation among a plurality ofspectral lines wherein differences in attenuation among various portionsof the radiation spectrum arise from selective atmospheric absorption ofradiation at various frequencies as a function of propagation distanceof the radiation through the atmosphere. The foregoing attenuation is inaddition to the attenuation arising from the spreading of the waves ofradiation through increasing regions of space, the latter attenuationfollowing the well-known relationship of intensity varying as theinverse square of the range from a point source of the radiation.

Various objects, such as the plume of a rocket or other fire, or a hotfilament or gas discharge of a lamp, are known to act as sources ofradiation having characteristic spectra. There are situations in whichit is desirable to determine the location of such a source from aviewing site distant from the source, the location data including range,elevation and azimuth of the target source from the viewing site.However, a problem arises in that the usual apparatuses fordetermination of target location, such as active radar, are notoperative with the foregoing type of radiant energy signal for a passivedetermination of the range of the source. A further problem arises whena target conceals its source of radiation or when the source ofradiation is inactive.

SUMMARY OF THE INVENTION

The aforementioned problem is overcome and other advantages are providedby a system and method of passive ranging, in accordance with theinvention, wherein a suitable target, or distant source of radiation, isidentified by its electromagnetic spectrum during a target acquisitionprocedure and, thereafter, the spectrum of the radiation is analyzed todetermine the effects of atmospheric attenuation on various parts of thespectrum. In the practice of the invention, prior knowledge of thespectrum, as emitted by the target, is employed in both the acquisitionand the analysis stages. The invention is particularly useful in thesituation wherein a source of radiation, on or near the ground,illuminates a cloud above the source, and a distant observer obtainsrange of the source by observation of radiation scattered from thecloud.

A typical spectrum includes both a continue distribution of spectralenergies in an emission band or in each of a plurality of emissionbands, as well as a line spectrum wherein individual ones of the linesare characteristic of certain constituent substances in a source of theradiation, such as the various gasses in a rocket plume. In accordancewith the theory of the invention, a source of radiation, such as arocket plume, emits radiation characterized by a known set of spectralemission lines and/or emission bands. The lines of the line spectrum, aswell as an amplitude profile of the continuous spectrum, are useful inidentifying the source of the radiation. Generally, the spectrum of areceived radiation signal will be shifted in frequency by a Dopplershift due to motion of the source, and there will be a broadening of oneor more of the spectral lines due to movement of the gasses andparticles thereof within the rocket plume. To identify the spectrum of areceived radiation signal automatically, as by use of a computer orother signal processor, the received spectrum may be correlated againstknown spectra from a set of previously stored spectra. The previouslystored spectra correspond to respective ones of known rocket plumes andother sources of radiation which may be of interest. A match is obtainedbetween the received spectrum and one of the known spectra, the matchserving to identify the source of the radiation. The correlation alsoindicates a frequency offset between the two matching spectra and,hence, is useful in providing the additional information of Dopplershift.

In accordance with a feature of the invention, a continuous portion ofthe received spectrum can be employed to determine range of a target,such as the plume of a rocket, independently of whether or not there beany Doppler frequency shift. Operation of the invention to obtain therange may be explained as follows. As the radiation propagates throughthe atmosphere from the source to optical receiving apparatus employedby the invention, there may be interaction between the radiation andvarious substances dependent on the frequency of the radiation. Theinteraction results in a relative attenuation of various spectralcomponents by the atmosphere as a function of frequency and a functionof distance of propagation of the radiation through the atmosphere.Thus, the attenuation is indicative of target range.

Measurement of the ratios of intensities of radiations at the variousspectral bands at a distance from the source will differ from the samemeasurements performed at the location of the source because of theselective absorption of the radiation at its various spectral bands. Inthe practice of the invention, a correlation is made between variationof an intensity ratio of any two spectral lines as a function ofdistance between source and the receiving apparatus. The range to thesource is thereby obtainable from spectrometric measurements of theradiation, computation of the intensity ratio, and association of thespecific range with a specific intensity ratio, or an average value ofranges obtained from sets of intensity ratios. A succession of rangemeasurements may be differentiated to obtain range rate.

Another aspect of the present invention uses reflected solar radiation,or other radiation source (e.g., tunable laser, search light) located ata known position in the atmosphere, to determine the range and/or rateof a target. In the case of using the sun as the source, a referencemeasurement of solar radiation is made at a spot adjacent (i.e.,proximal target) to an optical apparatus employing the presentinvention. The reflected solar radiation is also measured as areflection off the target (i.e., distal target). As the reflected solarradiation propagates through the atmosphere from the target to opticalreceiving apparatus employed by the invention, there may be interactionbetween the solar radiation and various substances dependent on thefrequency of the reflected solar radiation. The interaction results in arelative attenuation of various spectral components by the atmosphere asa function of optical frequency and a function of distance ofpropagation and look angle or orientation of the reflected solarradiation through the atmosphere. Thus, the attenuation is indicative oftarget range. In an alternative embodiment, because the position of theradiation source, adjacent spot, and atmosphere between the radiationsource and radiation source adjacent spot are known, the reflectedradiation from the adjacent spot can be calculated rather than measured.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

The aforementioned aspects and other features of the invention areexplained in the following description, taken in connection with theaccompanying drawing figures wherein:

FIG. 1 is a block diagram of an electrooptic system useful in thepractice of the invention;

FIG. 2 is a block diagram of a signal processor forming a part of thesystem of FIG. 1;

FIG. 3 is a stylized representation of the frequency spectrum ofelectromagnetic radiation, having both continuous and line spectralportions, emitted by a target at zero range with three significantfrequency components of the continuous spectral portion being identifiedby the letters A, B, and C;

FIG. 4 is a stylized graph representing the relative amplitudes of thefrequency components A, B, and C of FIG. 3 after the radiation haspropagated through a distance in clear air, the amplitudes of thespectral components having been attenuated by the environment;

FIG. 5 shows the ratio of amplitudes of the A component versus the Bcomponent, and the A component versus the C component of the graph ofFIG. 4 as a function of distance from the target;

FIG. 6 is a diagram of method steps in obtaining target range fromspectral data; and

FIG. 7 shows, diagrammatically, a viewing of target radiation reflectedfrom a cloud by the electrooptic system of the invention, wherein thesystem may be carried by a vehicle on the ground or an airborne vehicle;

FIG. 8 is a diagram indicating the use of solar radiation by the presentinvention; and

FIG. 9 is a stylized graph representing the relative amplitudes of thefrequency components A and B after the solar radiation of FIG. 8 hasreflected off the target.

Identically labeled elements appearing in different ones of the figuresrefer to the same element in the different figures.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

FIG. 1 shows an electrooptic system 10 for obtaining passively spectraldata of electromagnetic radiation emitted by a distant target 12. Inaccordance with the invention, the electromagnetic radiation emitted bythe target 12 has a known spectrum, or target signature, which is storedin a signature memory 14. The system 10 includes a telescope 16 whichviews electromagnetic radiation, indicated as a plurality of light rays18, which propagates through the atmosphere 20 to be incident upon thetelescope 16. The telescope 16 is steered mechanically in azimuth and inelevation by a scanner 22 which enables the telescope 16 to scan throughspace to determine whether the target 12 as well as other targets may bepresent. By way of example, the telescope 16 is shown in a Cassegrainform having a main mirror 24 and a secondary mirror 26, the latterreflecting light through an aperture 28 to an optical assembly 30.

The optical assembly 30 provides an optical path from the telescope 16to a signal processor 32 of the system 10. The signal processor 32,operates in a manner to be described with reference to FIG. 2, forextracting spectral data from the target radiation, and for determiningthe range of the target 12 to the telescope 16 from the spectral data.The optical assembly 30 comprises a collimating lens 34 for establishinga beam 36 of parallel rays suitable for operation of the signalprocessor 32. In addition, the optical assembly 30 comprises fourpartially reflecting mirrors 38, 40, 42, and 44 for tapping off portionsof the optical energy of the beam 36 to be used for purposes ofacquiring and tracking the target 12.

The system 10 further comprises three spectral line filters 46, 48, and50, three detectors 52, 54, and 56 of target radiation received by thetelescope 16, and a correlation unit 58. In operation, a portion of theoptical energy of the beam 36 is reflected by the mirror 38 via thefilter 46 to the detector 52, the detector 52 converting the opticalenergy to an electrical signal which is applied to the correlation unit58. In similar fashion, optical energy reflected by the mirror 40propagates via the filter 48 to the detector 54 to be converted to anelectrical signal which is applied to the correlation unit 58. Also,optical energy reflected by the mirror 42 propagates through the filter50 to be converted by the detector 56 to an electrical signal which isapplied to the correlation unit 58.

The filters 46, 48, and 50 provide different specific passbands for thepropagation of the optical energy of the beam 36. This enables each ofthe filters 46, 48, and 50, in conjunction with the respective detectors52, 54 and 56, to view only a specific portion of the spectrum of thetarget radiation while discarding the balance of the radiation. Thereby,the detectors 52, 54, and 56 signal the presence of specific spectrallines. The absence of a signal outputted by any one of the detectors 52,54, and 56 is an indication of the absence of the corresponding spectralline from the spectrum of the target radiation. It is to be noted thatthe use of three signal channels provided by the three mirrors 38, 40,and 42 in combination with the three filters 46, 48, and 50, and thethree detectors 52, 54, and 56 is presented by way of example and that,in practice, more of these signal channels may be employed forobservation of additional spectral lines of the target spectrum. Thecorrelation unit 58 obtains best fit between incoming spectral data,which may be Doppler shifted in the event of target motion, and theknown spectrum of the target radiation stored in the signature memory14. Thresholds, stored in a memory 60, are employed by the correlationunit 58 in a decision process of the correlation unit 58 for deciding ifa specific spectral line is considered to be present.

The system 10 includes a memory 62 for storing the locations of possibletargets in terms of azimuth and elevation address, a switch 64 operatedby the correlation unit 54, a Faraday filter 66, a detector assembly 68comprising an array of charge-coupled devices (CCD) providing atwo-dimensional viewing of target image data on the beam 36, atrack-mode electronics unit 70, and an acquisition-mode electronics unit72. In operation, optical energy extracted from the beam 36 by themirror 44 is directed by the mirror 44 via the Faraday filter 66 to thedetector assembly 68. The use of the Faraday filter 66 is well known,such use being described in an article entitled HELICOPTER PLUMEDETECTION BY USING AN ULTRANARROW-BAND NONCOHERENT LASER DOPPLERVELOCIMETER by S. H. Bloom et al, appearing in OPTICS LETTERS, Vol. 18,No. 3, Feb. 1, 1993 at pages 244-246.

The optical passband of the Faraday filter 66 is dependent on thestrength of the magnetic field of the filter, and a specific spectralregion of the incoming radiation may be selected for viewing via thefilter 66 by adjustment of the magnetic field strength. The magneticfield strength is set by a passband signal outputted by the correlationunit 58 corresponding to the detection of a desired spectral line by oneor more of the detectors 52, 54, and 56. The rays of light passingthrough the filter 66 retain their relative directions of orientation sothat the detector assembly 68 is able to determine whether the source ofthe target radiation appears to be above or below the boresight axis ofthe telescope 16, or to the right or the left of the boresight axis.Thereby, the detector assembly 68 provides an error signal to thetrack-mode electronics unit 70 which indicates whether the telescope 16is to be repositioned or oriented by the scanner 22 during a tracking ofthe target

The acquisition-mode electronics unit 72 is operative to provideelectric signals to the scanner 22 for directing the telescope 16 toview a designated portion of space during a scanning of space in theacquisition mode. The decision as to whether to enter the acquisitionmode or the tracking mode is made by the correlation unit 58. Initially,the switch 64 is in the acquisition position for coupling signals fromthe acquisition-mode electronics unit 72 to the scanner 22. During theacquisition process, any possible targets noted by the correlation unit58 are entered into the memory 62. This is accomplished by an outputsignal of the correlation unit 58 which strobes the memory 62 to storethe azimuth and elevation command signals outputted by theacquisition-mode electronics unit 72 to the scanner 22. The storage ofthe possible target locations in the memory 62 is useful for entering areacquisition mode wherein the electronics unit 72 scans a region ofspace around a possible target to ascertain the target coordinates inazimuth and in elevation.

Additionally, the acquisition-mode electronics unit 72 outputs thetarget coordinates to the track-mode electronics unit 70 during ahand-off procedure wherein the switch 64 is operated to disconnect theacquisition-mode electronics unit 72 from the scanner 22 and to connectthe track-mode electronics unit 70 to the scanner 22. This operation ofthe switch 64 occurs upon the determination by the correlation unit 58that a target is present. The azimuth and elevation (AZ/EL) coordinatesof the target being tracked are applied by the track-mode electronicsunit 70 to the signal processor 32, via line 74, for use in identifyinga specific target by its angular coordinates.

As shown in FIG. 2, the signal processor 32 comprises an addressgenerator 76 driven by a clock 78, and spectrum analyzer 80 whichreceives the beam 36 (FIG. 1) and is driven by a scanning drive 82. Thesignal processor 32 further comprises two memories 84 and 86 which areaddressed by the address generator 76. The memory 84 stores known targetspectral data for the target 12 (FIG. 1) as well as for other targetswhich may be viewed by the telescope 16 (FIG. 1). The memory 86 storesspectral data of the target 12 obtained by operation of the spectrumanalyzer 80. The address provided by the generator 76 is in terms of thefrequency coordinate in a graph of amplitude versus frequency for thetarget spectral data. The generator 76 is operative also to address thescanning drive 82 for directing the drive 82 to drive the spectrumanalyzer 80 to a specific frequency during a scanning of the spectrum.Operation of the drive 82 may be either mechanical or electricaldepending on the construction of the spectrum analyzer 80.

By way of explanation of the operation of the invention for themeasurement of range to a source of radiation, it is noted that in ahypothetical case, in the absence of selective atmospheric attenuationof various portions of the target spectrum, such as for the propagationof radiation in vacuum, it is apparent that the relative amplitudes ofvarious frequency components in the reference spectrum of the memory 84would be the same as those being measured by the spectrum analyzer 80.However, due to the presence of the atmosphere 20 (FIG. 1), theselective attenuation results in a distortion of the measured spectrumsuch that the relative intensities of the spectral lines differ betweenthe measured and the reference spectra. The nature of the distortiondepends on the propagation distance of the radiation through theatmosphere. The invention employs a relatively small continuous portionof the electromagnetic spectrum wherein the influence of clouds,rainfall, aerosols, or dust can be discounted because they present asubstantially uniform attenuation, as a function of frequency, acrossthe small portion of the spectrum employed for the range measurement.

With a knowledge of the atmospheric attenuation rates as a function ofdistance at various frequencies of the spectrum, the signal processor 32can derive the target range by analysis of distortion in the receivedspectrum as compared to the reference spectrum. Assuming that thecontinuous portion of the target spectrum, utilized in the measurement,is essentially constant in amplitude at zero range, before attenuationby the atmosphere, a measurement of spectral distortion from attenuationcan be accomplished without regard to Doppler frequency shift. The useof the continuous spectrum avoids any effect of a broadening of spectrallines by collisions among particles in the constituent substances of arocket plume.

In the event that the nature of the target 12 is unknown, or in theevent that any one of a plurality of targets (not shown) may be present,it is useful to provide means for identification of the target 12. Inorder to identify the target 12, the signal processor 32 furthercomprises a correlator 88 which correlates measured spectral data storedin the memory 86 with the known spectral data stored in the memory 84 todetermine if a match can be made. Identification of the type of targetis made by use of the target spectrum as a signature. A match betweenthe spectra identifies the nature of the source of radiation, such asthe plume of a rocket, and thereby serves to identify the target 12. Forexample, the spectrum may indicate a combustion of a certain type offuel which serves to identify the target.

Also included in the signal processor 32 is computational equipment forcalculation of target range, as well as for utilization of the targetrange to calculate range rate and trajectory. The computationalequipment, for convenience in explaining operation of the processor 32,is portrayed as three separate computers 94, 96, and 98. The computer 94receives input signals from the memories 86 and 84, and also receivesatmospheric data stored at 100 in order to compare various ratios ofintensities of selected spectral components of the measured targetspectrum with corresponding ratios of intensities of the targetreference spectrum. This will be described hereinafter in greaterdetail.

The range of the target produced by the computer 94, and the identity ofthe target produced by the correlator 88 are applied to the computer 96.Range rate can then be computed by the computer 96 by observing changesin range over an interval of time. Target range, range rate, andidentity are then outputted by the computer 96 to the computer 98. Thecomputer 98 receives the target azimuth and elevation coordinates vialine 74 and, in conjunction with the range and the range rate, computestarget trajectory. The azimuth and elevation coordinates also serve toidentify the target by location. The target identity and trajectory dataare outputted by the computer 98 to a display 102 for outputting datarelative to each of the targets selected by the correlation unit 58(FIG. 1) for analysis. The display 102 may include recording apparatus(not shown) for recording the data.

By way of example, in the use of the spectrum analyzer, 80, emissionlines of sodium and potassium are discerned readily in the hot plumes ofrockets by atomic line filters, such as the filters 46, 48, and 50(FIG. 1) and by the spectrum analyzer 80. Such spectral lines facilitateidentification of the target. The use of a spectrum analyzer, such asthe analyzer 80, is disclosed in the aforementioned article of S. H.Bloom et al. The sodium and the potassium spectral lines are presentedby way of example, and numerous other lines may be observed, dependingon chemistries of the sources of radiation. In atomic spectroscopy,there are well-known doublet lines appearing in the spectrum which alsoserve to identify a source of the radiation. It is also recognized thatthe effect of atmospheric attenuation may vary with elevation angle and,accordingly, in FIG. 2, the target coordinates on line 74 are appliedalso for addressing the atmospheric data store 100 to select anatmospheric attenuation profile consonant with a specific value oftarget elevation.

FIGS. 3-5 demonstrate attenuation of various frequency components oftarget radiation as a function of distance from the target. By way ofexample, FIG. 3 shows a stylized representation of the radiation whereinthe spectrum of the radiation has both a continuous portion and a set ofspectral lines. This is the spectrum which would be measured at thelocation of the target, namely, at zero range. In the continuous portionof the spectrum, three components are identified by the legends A, B,and C by way of example. The frequency of component B is greater thanthe frequency of component A, and the frequency of component C isgreater than the frequency of component B. In FIG. 4, the spectrum hasbeen simplified to show only the continuous portion with the componentsA, B, and C. FIG. 4 depicts relative attenuation of the components A, B,and C at a distance from the target of five kilometers, by way ofexample.

Upon comparing the graphs of FIGS. 3 and 4, it is noted that in FIG. 3,the amplitudes of the components A, B, and C are equal. In FIG. 4,component B is substantially smaller than component A, and component Cis substantially smaller than component B due to atmospheric absorption.Thus, upon comparing the amplitudes of the various components, such asthe ratio of A to B, and the ratio of A to C, by way of example, it isobserved that these ratios, which are unity in FIG. 3, differ markedlyin FIG. 4.

Due to the selective attenuation of the radiation by the atmosphere as afunction of frequency, the intensity ratio changes with increasingdistance from the target. This is portrayed in FIG. 5 wherein the linesrepresent the amplitude ratios A/C and A/B. The attenuation of theradiation by the atmosphere is described by the slopes of the graph ofFIG. 5, the slopes being determined by the atmospheric attenuationfactors. Information from the memory 84 (FIG. 2) is employed by thecomputer 94 (FIG. 2) to compute the attenuation factors. The attenuationfactors are based on experimental evidence as is stored in the memory 84wherein spectra are stored for measurements conducted at variousdistances from each of a plurality of radiation sources. The computer 94then computes the amplitude ratios of the selected frequency componentsof the measured continuous spectrum from the memory 86, and employs theamplitude ratios of the frequency components to compute the targetrange.

Calculation of the target range can be accomplished by (1) establishingan initial value of the amplitude ratios of the essential spectralcomponents at zero range as is depicted in the left side of FIG. 5, (2)establishing the ratio of the spectral components at a nonzero distancesuch as at five kilometers presented in FIG. 5, (3) establishing theslope of a specific graph of FIG. 5 from prior knowledge of atmosphericattenuation, and (4) solvine mathematically the graphically portrayedrelationship of FIG. 5 for the propagation distance along the horizontalaxis of FIG. 5.

FIG. 6 outlines the essential steps of the foregoing procedure wherein,in block 104 the weather or other atmospheric condition is noted, and atblock 106 the spectrum of a suitable target is identified. The weatherconditions of block 104 is stored in the atmospheric data storage 100(FIG. 2), and the identification of a suitable target spectrum isobtained during the acquisition and track modes of FIG. 1, by operationof the correlation unit 58. At block 108, a selection is made ofspectral lines to be used in forming the ratios, the selection beingmade by the computer 94 (FIG. 2) which uses, by way of example, thestrongest spectral lines, such as the three strongest spectral lines inthe continuous spectrum of FIG. 3. Then, at block 110, the amplituderatios of the spectral lines are formed for the measured spectrum storedin the memory 86 (FIG. 2). The ratios are compared at block 112, thecomparing being done in the computer 94. This is followed by acomputation of the range at block 114. The computation of the range isaccomplished by the computer 94 following the procedure outlined abovewith reference to the graphs of FIG. 5. Thereby, the invention hasaccomplished the attainment of target range by a passive observation ofradiation emitted by the target. It is noted that the foregoingmeasurement of rang can be accomplished by use of a value of intensityratio obtained from the spectral components A and B or from the spectralcomponents A and C. Alternatively, in the practice of the invention,plural intensity ratios can be employed for improved accuracy ofmeasurement. For example, in the operation of the computer 94, a firstmeasurement of the range can be accomplished by use of the spectralcomponents A and B, and a second measurement of the range can beaccomplished by the use of the spectral components A and C. The twomeasurements are then averaged by the computer 94 to provide an averagevalue of the range measurement for improved accuracy in thedetermination of the range. As has been noted above, the graphs of FIGS.3-4 depict the three spectral lines, A, B, and C, by way of example, andthat additional lines such as lines D and E (not shown) may be employedfor determination of still further intensity ratios for yet additionalmeasurement of range. Any pairs of frequencies, such as the ratio of Band C, or the ratio of C and D, and the ratio of A and D, may beemployed, assuming that the ratios are statistically independent. Thus,three spectral lines provide two statistically independent ratios, andfour spectral lines provide three statistically independent ratios, byway of example. In this manner, the use of plural ratios from both thereceived and the reference spectra may be employed for improved accuracyin the determination of the range.

In the foregoing analysis, it has been assumed that the spectralcomponents of the continuous spectrum have equal amplitude. In the eventthat these spectral components differ in amplitude, such amplitudedifference appears in the reference spectrum provided by the memory 84to the computer 94. In such case, the computer 94 introduces anadditional multiplicative factor to each of the amplitude ratios of theselected frequency components to compensate for the differences ofamplitude in the spectral components at zero range.

With reference to FIG. 7, the system 10 may view radiation from thetarget 12 in a situation, wherein the target 12 is located beyond theearth's horizon, by observation of radiant energy emitted by the target12 and reflected from a cloud 116 via rays 18 of radiation. Typically,the system 10 is located on the earth's surface, as indicated in solidlines, or is provided as an airborne system 10′ carried by an aircraft120, as indicated in phantom view. FIG. 7 shows the situation whereinthe target 12 is a rocket 122 emitting a plume 124 which is a source ofradiation 126 reflected via the rays 118 from the cloud 116 to be viewedby the system 10, or the system 10′.

FIG. 8 is a diagram of an application in which an embodiment of thepresent invention passively determines a range to a target reflectingsolar radiation or other suitable radiation source, such as a searchlight or tunable laser. In this application, the target need not beself-luminous since the solar radiation is providing the electromagneticspectrum used during a target acquisition.

In the case of using the sun as the source of the radiation, theposition of the sun 150 is static within the time frame of themeasurement. The sun 150 illuminates a target 154 and a spot 156adjacent to a sensor (i.e., optical assembly employing the presentinvention) at a known position, for example, on a tank 158. Solarradiation 152 reflects off the target 154 and is represented asreflected radiation 160 a. Solar radiation 152 further reflects off theadjacent spot 156 and is represented as reflected radiation 160 b. Thesensor in the tank 158 measures the reflected radiation 160 a and 160 band calculates a differential attenuation between the two spectra.

The tank 158, employing the principles of the present invention,compares the absorption spectrum off the adjacent spot 156 (e.g.,ground) with the reflected radiation 160 a off the target 154, thus,using Beer's Attenuation Law and known attenuation of the atmosphere. Ifoperating at higher elevations, consideration for the lower absorptionof the higher atmosphere must be taken into account. It should beunderstood that since the sun 150 is illuminating the adjacent spot 156,and the sun 150 and adjacent spot 156 both are at known positions, it ispossible for the present invention to calculate the solar radiationspectra rather than measuring it. If, however, measurement of theadjacent spot 156 is to be made, a small flip mirror may be employed inthe optical assembly apparatus to switch the look angle between thetarget 154 and the adjacent spot 156.

It is desirable for the differential attenuation between the reflectedradiation from the adjacent spot 160 b and the reflected radiation 160 afrom the target 154 to be large. A large differential attenuationresults in more sensitivity than if the differential attenuation weresmall.

FIG. 9 provides a graph indicating relative amplitudes between spectraof the reflected radiation 160 b from the adjacent spot 156 and thereflected radiation 160 a from the target 154. The wavelengths ofinterest fall between λ₁ and λ₂. At λ_(A), the range from the adjacentspot 156 (unabsorbed radiation) to the target 154 (absorbed radiation)is determined as a function of distance and angle, F(R, ζ, φ). Relativeattenuation of the components A and B can be determined in a manner setforth above. Further, rate and angle can also be determined in a manneras set forth above.

It is to be understood that the above described embodiment of theinvention is illustrative only, and that modifications thereof may occurto those skilled in the art. Accordingly, this invention is not to beregarded as limited to the embodiment disclosed herein, but is to belimited only as defined by the appended claims.

What is claimed is:
 1. A system for passive determination of the range of a source of known spectral emission, comprising: optical apparatus for receiving radiation propagating from the source via a first atmospheric propagation path to the optical apparatus, the atmosphere providing for selective attenuation of spectral lines of the radiation as a function of frequency of the spectral lines; said optical apparatus for further receiving reflected radiation reflected by a reflective object in a path of radiation from the source via a second atmospheric propagation path to the reflective object; computer means, and means operatively coupled to said computer means for storing a known spectrum of the radiation as emitted by the source, at least a portion of said known spectrum being continuous with substantially constant amplitude and having at least a first frequency component and a second frequency component; means operatively coupled to said computer means for analyzing a received spectrum of the radiation as received by said first and second optical apparatus, said received spectrum having at least a first frequency component and a second frequency component; wherein said first and said second frequency components of said received spectrum are alterable from said first and said second frequency components of said known spectrum a distance between said system and said reflective object; said system further comprises means for providing said computer means with spectrally dependent attenuation characteristics of the atmosphere; said computer means computes plural amplitude ratios of frequency components wherein one of said amplitude ratios is the ratio of amplitudes of the first and the second frequency components of said known spectrum and a second of said amplitude ratios is the ratio of amplitudes of the first and the second frequency components of said received spectrum; and said computer means is operative further to determine the range of the reflective object based on said amplitude ratios and on said attenuation characteristics of the atmosphere.
 2. A system according to claim 1 further comprising means coupled to said computer means for computing range rate of said reflective object from a succession of range measurements of the reflective object.
 3. A system according to claim 1 wherein said computer means is operative to correlate the received spectrum with the known spectrum for identification of the reflective object.
 4. A system according to claim 1 wherein said optical apparatus includes a signature memory and means for correlating the received spectrum with a reference spectrum of said signature memory to determine the presence of the reflective object.
 5. A system according to claim 1 wherein said reflective object undergoes motion relative to said optical apparatus, and said optical apparatus includes means for tracking the reflective object.
 6. A system according to claim 1 wherein the determination of the range of the reflective object constitutes a first range measurement; said known spectrum includes a third frequency component and said received spectrum includes a third frequency component; and said computer means is operative to provide an additional measurement of the range of the reflective object based on the third frequency component of said known spectrum and the third frequency component of said received spectrum, said computer means being operative further to provide an average value of the first range measurement and the additional range measurement.
 7. A method for passive determination of the range of a source of known spectral emission by use of optical apparatus for viewing the source, comprising the steps of: receiving radiation propagating from the source via a first atmospheric path to the optical apparatus, the atmosphere providing for selective attenuation of frequency components of the radiation as a function of frequency of the frequency components; further receiving reflected radiation reflected by a reflective object in a path of radiation from the source via a second atmospheric propagation path to the reflective object; storing a known spectrum of the radiation as emitted by the source, at least a portion of said known spectrum being continuous with substantially constant amplitude and having at least a first frequency component and a second frequency component; analyzing a received spectrum of the radiation as received by the optical apparatus, the received spectrum having at least a first frequency component and a second frequency component; wherein said first and said second frequency components of said received spectrum are alterable from said first and said second frequency components of said known spectrum a distance between said optical apparatus and said reflective object; said method further comprising steps of obtaining spectrally dependent attenuation characteristics of the atmosphere; computing plural amplitude ratios of the frequency components wherein one of said amplitude ratios is the ratio of amplitudes of the first and the second frequency components of said known spectrum and a second of said amplitude ratios is the ratio of amplitudes of the first and the second frequency components of said received spectrum; and determining the range of the reflective object based on said amplitude ratios and on said attenuation characteristics of the atmosphere.
 8. A method according to claim 7 wherein said known spectrum includes a third frequency component and said received spectrum includes a third frequency component, said method including further steps of determining range based on computation of additional amplitude ratios having said third frequency components to produce the range of the reflective object; and averaging the range of the reflective object provided in both of said range-determining steps.
 9. A system for passive determination of the range of an object, comprising: optical apparatus for receiving radiation propagating from a source of known spectral emission via an atmospheric propagation path to the optical apparatus, the atmosphere providing for selective attenuation of spectral lines of the radiation as a function of frequency of the spectral lines; said optical apparatus for further receiving reflected radiation reflected by a reflective object in a path of radiation from the source via a second atmospheric propagation path to the reflective object; at least one computer, and a data storage medium operatively coupled to said at least one computer that stores known atmospheric attenuation data for at least a first frequency component and a second frequency component, said first frequency component being attenuated by propagating through a known distance in the atmosphere, said second frequency component being attenuated in a lesser amount by propagating through the same distance in the atmosphere; a spectrum analyzer operatively coupled to said at least one computer that analyzes a received spectrum of the radiation as received by said optical apparatus, said received spectrum having at least a first frequency component and a second frequency component; wherein said first and said second frequency components of said received spectrum are alterable from said first and said second frequency components of said known spectrum a distance between said system and said reflective object; said system further comprises an atmosphere information unit that provides said at least one computer with spectrally dependent attenuation characteristics of the atmosphere; said at least one computer computes plural amplitude ratios of said frequency components wherein one of said amplitude ratios is the ratio of amplitudes of the first and the second frequency components of said known spectrum and a second of said amplitude ratios is the ratio of amplitudes of the first and the second frequency components of said received spectrum; and said at least one computer is operative further to determine the range of the reflective object based on said amplitude ratios and on said attenuation characteristics of the atmosphere.
 10. The system according to claim 9, further comprising a range rate calculation unit coupled to said at least one computer that computes range rate of said reflective object from a succession of range measurements of the reflective object.
 11. The system according to claim 9, wherein said at least one computer is operative to correlate the received spectrum with the known spectrum for identification of the reflective object.
 12. The system according to claim 9, wherein said optical apparatus includes a signature memory and a correlator to correlate said received spectrum with a reference spectrum of said signature memory to determine the presence of the reflective object.
 13. The system according to claim 9, wherein said reflective object undergoes motion relative to said optical apparatus, and said optical apparatus includes a tracking unit to track the reflective object.
 14. The system according to claim 9, wherein: the determination of the range of the reflective object constitutes a first range measurement; said known spectrum includes a third frequency component and said received spectrum includes a third frequency component; and said at least one computer is operative to provide an additional measurement of the range of the reflective object based on the third frequency component of said known spectrum and the third frequency component of said received spectrum, said at least one computer being operative further to provide an average value of the first range measurement and the additional range measurement.
 15. A system for passive determination of the range of an object, comprising: optical apparatus to receive radiation propagating from a source of known spectral emission via an atmospheric propagation path to the optical apparatus, the atmosphere providing for selective attenuation of spectral lines of the radiation as a function of frequency of the spectral lines; said optical apparatus to further receive reflected radiation reflected by a reflective object in a path of radiation from the source via a second atmospheric propagation path to the reflective object; said optical apparatus further comprising: at least one wide-band optical detector to convert, from optical energy to a spectral portion electrical signal, a portion of the received spectrum of the radiation from at least the reflective object as received by the optical apparatus, said portion of the received spectrum having at least a first frequency component and a second frequency component; first and second narrow-band optical detectors, to convert from optical energy to respective first and second narrow-band electrical signals, the radiation propagating from at least the reflective object to the optical apparatus, said first narrow-band optical detector receiving through a respective spectral line filter said first frequency component, but not said second frequency component, and said second narrow-band optical detector receiving through a respective line filter said second frequency component, but not said first frequency component; a spectrum analyzer unit coupled to said at least one wide-band optical detector to analyze the spectral portion electrical signal, said spectrum analyzer further coupled to said first and second narrow-band optical detectors and being activated only upon an indication based on the first and second narrow-band electrical signals of the presence of said first and second frequency components; a computer, and computer readable storage media operatively coupled to said computer, said computer readable storage media storing a known spectrum of the radiation as emitted by the source, at least a portion of said known spectrum being continuous with substantially constant amplitude and including at least said first frequency component and said second frequency component; said computer computing plural amplitude ratios of frequency components wherein one of said amplitude ratios is the ratio of amplitudes of the first and second frequency components of said known spectrum and a second of said amplitude ratios is the ratio of the first and second frequency components of said spectral portion; and said computer is operative further to determine the range of the reflective object based on said amplitude ratios and on said attenuation characteristics of the atmosphere.
 16. The system according to claim 15, further including a correlation unit coupled to said first and second narrow-band optical detectors, said correlation unit obtaining a best fit between incoming spectral data of the reflected radiation propagating from the reflective object and the known spectrum of said source radiation stored in said computer readable media.
 17. The system according to claim 16, further including a Faraday filter having an optical passband dependent on the strength of a magnetic field of said Faraday filter, the magnetic field strength being set by a passband signal being outputted by the correlation unit corresponding to the detection of a spectral line by at least one of the narrow-band optical detectors.
 18. The system according to claim 17, further including: a detector assembly comprising an array of charge-coupled devices providing a two-dimensional viewing of a portion of the reflected radiation; and an acquisition-mode electronics unit electrically coupled to said detector assembly, said detector assembly providing a boresight error signal to said acquisition-mode electronics unit, which, in turn, provides electrical signals to said optical apparatus for repositioning.
 19. The system according to claim 15, wherein said first and second frequency components of said received spectral portion of the reflected radiation are alterable from said first and said second frequency components of said known spectrum by Doppler frequency shift due to motion of said reflective object.
 20. The system according to claim 15, further comprising an atmosphere information unit that provides said at least one computer with spectrally dependent attenuation characteristics of the atmosphere.
 21. The system according to claim 15, wherein said continuous substantially constant spectral portion enables said ratios to be independent of Doppler frequency shift.
 22. The system according to claim 15, further comprising a range rate calculation unit coupled to said at least one computer for computing range rate of said reflective object from a succession of range measurements of the reflective object.
 23. The system according to claim 15, wherein said at least one computer is operative to correlate the received spectral portion of the reflected radiation with the known spectrum for identification of the reflective object.
 24. The system according to claim 15, wherein said optical apparatus includes a signature memory and a correlator to correlate said received spectrum of the reflected radiation with a reference spectrum of said signature memory to determine the presence of the reflective object.
 25. The system according to claim 15, wherein said known spectrum includes a third frequency component and said received spectrum of the reflected radiation includes a third frequency component, said at least one computer further being employed to: determine range based on computation of additional amplitude ratios having said third frequency components to produce the range of the reflective object; and average the range of the reflective object provided in both of said range-determining steps.
 26. A method for passive determination of an object, comprising: receiving radiation propagating from a source of known spectral emission via an atmospheric propagation path to an optical apparatus, the atmosphere providing for selective attenuation of spectral lines of the radiation as a function of frequency of the spectral lines; receiving reflected radiation reflected by a reflective object in a path of radiation from the source via a second atmospheric propagation path to the reflective object; storing a known spectrum of the radiation as emitted by the source, at least a portion of said known spectrum being continuous with substantially constant amplitude and having at least a first frequency component and a second frequency component; converting, from optical energy to a spectral portion electrical signal, a portion of the received spectrum of the reflected radiation reflected by the reflective object via the second atmospheric propagation path to the optical apparatus by a wide-band optical detector in said optical apparatus, said portion of the received spectrum corresponding to said spectral portion being continuous with substantially constant amplitude and having at least said first frequency component and said second frequency component; converting, from optical energy to first and second narrow-band electrical signals, the reflected radiation reflected by the reflective object to the optical apparatus by first and second narrow-band optical detectors in said optical apparatus, said first narrow-band optical detector receiving through a respective spectral line filter said first frequency component, but not said second frequency component, and said second narrow-band optical detector receiving through a respective spectral line filter said second frequency component, but not said first frequency component; analyzing the spectral portion electrical signal corresponding to the portion of the received spectrum of the reflected radiation converted by the wide-band optical detector in response to determining that said first and second frequency components are present in the received spectral portion of the reflected radiation; computing plural amplitude ratios of frequency components wherein one of said amplitude ratios is the ratio of amplitudes of the first and second frequency components of said known spectrum and a second of said amplitude ratios is the ratio of the first and second frequency components of said spectral portion of the reflected radiation; and determining the range of the reflective object based on said amplitude ratios and on said attenuation characteristics of the atmosphere.
 27. The method according to claim 26, further including obtaining a best fit between incoming spectral data of the reflected radiation by the reflective object and the known spectrum of said source radiation.
 28. The method according to claim 26, wherein said first and second frequency components of said received spectrum of reflected radiation are alterable from said first and said second frequency components of said known spectrum by Doppler frequency shift due to motion of said reflective object.
 29. The method according to claim 26, further comprising steps of obtaining spectrally dependent attenuation characteristics of the atmosphere.
 30. The method according to claim 26, wherein said continuous substantially constant spectral portion enables said ratios to be independent of Doppler frequency shift.
 31. The method according to claim 26, wherein said known spectrum includes a third frequency component and said received spectral portion of the reflected radiation includes a third frequency component, said method further including: determining range based on computation of additional amplitude ratios having said third frequency components to produce the range of the reflective object; and averaging the range of the reflective object provided in both of said range-determining steps. 